1. Field of the Invention
This invention relates to a rust preventive treatment method for an aluminum basis material such as an outboard motor body or the like made of aluminum or aluminum alloy, and to an outboard motor body which is treated by this rust preventive treatment method.
2. Description of the Related Art
Conventionally, it is normal practice to construct a body of an outboard motor by a die-cast aluminum alloy such, for example, as AlADC 12 of JIS/H 5302 (which is equivalent to ASTM SC 114A, i.e., consisting of 1.5-3.5% Cu, 10.5-12.0% Si, less than 0.3% Mg, less than 1.0% Zn, less than 1.3% Fe, less than 0.5% Mn, less than 0.5% Ni, less than 0.3% Sn, and Al for the remaining percent). This alloy is likely to be easily corroded because it contains about 2 to 3% of copper. Therefore, conventionally it is practiced in general, as shown in FIG. 3, to form on a surface of a diecast basis material a for the outboard motor body, a chromate film b by treatment with chromic acid for rust prevention, and on top thereof a three-layer paint coat comprising a primer c containing a rust preventive pigment consisting of zinc chromate, a top coat d and a clear coat e.
The above-described chromate film b is as thin as about 10 to 100 angstroms, and the basis material a for the outboard motor body is easily exposed by scratches or the like. In addition, since this chromate film b is susceptible to heat, cracks are likely to be generated at the time of baking of the paint. As a consequence, rusts are formed in 500 to 600 hours in salt spray test.
In the Japanese Published Examined Patent Application No. 4155/Showa 56 (1981) and others, an art is disclosed in which an anodic oxide film is formed on a surface of an aluminum basis material made of aluminum or aluminum alloy in order to improve its wear resistivity, and then the anodic oxide film is sealing-treated with molybdenum disulfide in order to provide the film with lubricating characteristics.
Although the above-described art was originally developed for those mechanical parts of aluminum make which require lubricating characteristics, the inventor of this invention has found that the corrosion resistivity of aluminum basis material remarkably improves by forming an anodic oxide film which is sealed with molybdenum disulfide.
An object of this invention is to provide, based on this finding, a rust preventive treatment method for aluminum basis material. Another object of this invention is to provide an outboard motor body which is treated by the above-described rust preventive treatment method and which is superior in corrosion resistivity.